Move validateContextDirectory to builder package.

This feels like it's where it belongs and it makes it exported
again (which is needed for libcompose that was using it before 1.10).

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

builder/context_unix.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package builder
+
+import (
+	"path/filepath"
+)
+
+func getContextRoot(srcPath string) (string, error) {
+	return filepath.Join(srcPath, "."), nil
+}